1. Field of the Invention
The subject invention is related to proximity sensors for detection of a position of an object, and more particularly, to a wireless proximity sensor and sensing system that uses surface acoustic wave (SAW) technology to reduce weight and complexity to meet the rigorous demands of the aviation industry and other position sensing applications.
2. Description of the Related Art
Proximity sensors have long been used to indicate the position of an object on an aircraft or other vehicle (e.g., whether a door or hatch is open or closed). These proximity sensors are typically the current sensing induction type and heavily armored for isolation. The proximity sensor generates an electromagnetic field to sense metal objects passing within a few tenths of inches.
The heavy isolation armor or shielding and wire required are highly undesirable in the field of weight sensitive aircraft design. By one estimate for fixed wing commercial aircraft, each extra pound has a recurring cost of over U.S. $500, a non-recurring cost of over U.S. $5000, and a life cycle cost of U.S. $100,000. For rotary wing craft (e.g., helicopters), the costs are typically twice that of fixed wing aircraft. Yet, a Boeing 787 airplane has approximately 150 proximity sensors with over 40 pounds of associated wire.
While the aviation industry is looking for alternatives, the currently available options have their own disadvantages. One alternative is a voltage sensing capacitive type sensor, which may require heavy isolation hardware as a result of capacitive coupled voltage due to electromagnetic interference. Conductive fluids and water condensation pose additional problems for capacitive type sensors. Magnetic or optical interrupt switches, although much lighter due to less shielding, are failure prone in the harsh conditions of aviation. Further, magnetic switches attract ferrous metal filings that inhibit the detection of a near/far condition, resulting in false condition reporting and low reliability. Optical switches suffer from foreign object contamination, such as dirt, which again reduces the performance of the device. Further yet, each of these sensors uses hardwired connections to power and report state. Wire requires a number of aircraft interactions for installation that drives up cost in both design and manufacturability. Additionally, wire is heavy and difficult to troubleshoot when sensor operation fails.
SAW devices having a magnetostrictive element are one type of electronic component that may be used to address the problems of weight sensitive proximity detection of objects in aircraft and other applications. Generally, SAW devices generate guided acoustic waves along a surface of the device. SAW devices are typically fabricated on single crystal anisotropic substrates that are also piezoelectric. SAW devices typically include one or more pairs of intertwined interdigital electrodes that form transducers (known as an interdigital transducer or IDT) to convert the electrical signals applied to the device into the electromechanical surface acoustic waves generated in the device and vice versa. However, SAW devices may also be present in different classes of acoustic devices such as micro-strip couplers and acoustic reflectors/mirrors etc., depending on the particular device configuration employed.
One such SAW device is disclosed in U.S. Pat. No. 4,078,186 which discloses a magnetically tuned SAW device having a thin magnetostrictive film deposited only on the surface between its input and output transducers. A variable DC magnetic field is applied to the film by connecting the poles of an electromagnet to a DC supply. The electromagnet is then placed in line with the plane of the film to vary its magnetic field characteristics. A corresponding continuous variance in the delay or phase shift of the surface acoustic wave is produced thereby. This configuration however, has several drawbacks that do not make it ideal for proximity sensing applications. These drawbacks include, but are not limited to, the stationary positioning of the electromagnet and the weight of the wiring necessary to supply DC current to the poles of the electromagnet.
The present invention discloses a number of improvements over this and other known SAW devices and methods for proximity sensing, the features and advantages of which are described herein.